Extensible Markup Language (XML) is a markup language which was developed by the World Wide Web Consortium (W3C). A traditional XML document has an .xml file extension, and typically includes XML tags which are defined by the document's author. Such a document enables the author to describe data in a manner that complements, but does not replace, Hyper Text Markup Language (HTML) which is commonly used for web pages.
In a typical XML document, opening and closing tags delimit elements. In particular, each XML document contains a root element, and other elements nested within this root element. Furthermore, parent elements can have child (or sub) elements which are nested within these parent elements.
A variety of tools exist which enable the author to navigate through an XML document. For example, the XML Document Object Model (XML DOM) defines a standard way to access and manipulate XML documents. In particular, the XML DOM presents an XML document as an inverted tree having elements, attributes and text defined as nodes of the inverted tree. The author is capable of traversing and accessing components within the XML document using a set of standard functions.